In producing molten steel, a steel mill normally employs either a basic oxygen furnace or an electric arc furnace. When using a basic oxygen furnace, molten pig iron and solid steel scrap are converted into steel of the desired carbon content and temperature using high purity oxygen. The furnace or converter is a barrel shaped, open topped, refractory lined vessel that can rotate about a horizontal trunnion axis. The operational steps of the process involve charging scrap into the vessel, followed by charging of hot molten pig iron onto the scrap. When charging is complete, oxygen is blown through a lance to remove silicon, carbon, and phosphorous from the molten pig iron. The scrap melts during the process, commingling with the molten pig iron to form a bath of liquid steel. This process raises the temperature of the liquid steel made from the scrap and hot metal to above 2850° F. Flux is charged into the vessel for the purpose of combining with the silicon and phosphorous originating from the molten pig iron, thereby forming a molten slag. This slag also absorbs sulfur originating from the molten pig iron and steel scrap. Scrap is an important constituent of the basic oxygen process because, as the scrap melts it absorbs the heat generated by the oxidation of carbon contained in the molten pig iron. In an electric arc furnace, it is quite common to use bales of steel scrap as the source of iron units to be melted. In both a basic oxygen process and an electric arc process, the use of steel scrap presents processing difficulties. Compacted bales of steel scrap with variable compositions are shipped to the steel mill from many locations.
Depending on the source of the scrap, there are different levels of sulfur in the scrap and there is no technique to determine exactly how much sulfur is in the steel of a compressed scrap bale. On the other hand, it is standard practice to desulfurize molten pig iron before it is used for steel making.
In a basic oxygen furnace, the scrap is about 25-30% of the charge. Large pieces of scrap, such as compressed bales, which are preferred for logistics and handling, melt at the end of the process. When the steel scrap is completely melted, the sulfur contained in the steel scrap is released into the bath. Removing this sulfur with the use of standard fluxes requires a substantial amount of time to assure that the molten metal is sufficiently desulfurized for subsequent use in steel making. Thus, using bales of scrap delays the steel making process by requiring a longer processing time for sulfur removal in the melting furnace. Increased time is costly and constitutes a disadvantage of using steel scrap. The steel mill desires to desulfurize the molten steel as fast as possible so that the molten metal is ready for use in a very short time. Ideally, the molten steel should be removed from the furnace as soon as the molten metal has reached the desired tapping temperature. If the sulfur content of the steel bath is too high at tap, then it must be removed downstream of the vessel in the steel ladle at significant expense with a large negative impact on productivity. There is a need for improving the melting process using bales of scrap steel so that the molten metal is rapidly desulfurized to allow tapping the molten metal essentially as soon as the steel scrap has melted. This can not be done with the present technology since significant amounts of sulfur are locked in the solidified steel of the scrap.